No one knows the answer. It's unexplainable and often happens against the odds. The Shack or the erotic-romance series, 50 Shades of Grey, are two books that defied the odds.
Or think about dyslexic Debbie Macomber, a woman with no college education. She wrote a romance that was publicly critiqued by an editor from Harlequin at a romance writers conference. The editor trashed the novel and told her to throw it away.
Instead, Macomber sent her novel to a rival romance publisher. They bought it, and it was the first romance novel that Publishers Weekly reviewed. Her books now sell in the millions.
Why Debbie Macomber? It's unexplainable—it happens often enough to keep giving writers hope and encouragement.